1. Field of the Invention
The present invention relates to wireless communications, and more particularly to a method of transmitting data for reducing interference in a hierarchical cell structure.
2. Related Art
Along with development of communications and a spread of multimedia technology, various large-capacity transmission technologies have been applied to a wireless communication system. A method of assigning more frequency resources has been introduced as a method for increasing wireless capacity. But there is a limit to assign more frequency resources among restricted frequency resources to many users. One of methods for more efficiently utilizing the restricted frequency resources, there is a method of making a small cell. If the size of the cell is made small, the number of users one base station has to give service is decreased, so that the base station can assign more frequency resources to users. As the size of the cell is made small, it is possible to offer better large-capacity service to many users.
In an institute of electrical and electronics engineers (IEEE) 802.16 task group that chooses the next-generation air interface standards and a nonprofit worldwide interoperability for microwave access (WiMAX) forum that provides service and network standards for a broadband wireless connection system based on IEEE 802.16, standardization has been being achieved for a wireless connection system supporting a femto-cell to enhance efficiency of the system and improve quality of service (QoS) under an indoor environment. The WiMAX forum defines a femto base station as a low-power inexpensive base station connected to an Internet protocol (IP) network through a fixed wireless link or a local broadband wired link. The femto base station is connected to the IP network spread in a home or an office, and accesses a core network of a mobile communication system through the IP network, thereby providing mobile communication service. That is, the femto base station may be connected to the core network of the mobile communication system through a digital subscriber line (DSL). A user of the mobile communication system may receive the service in the outdoors through the existing macro-cell, but receive the service in the indoors through the femto-cell. The femto-cell improves indoor coverage of the mobile communication system by making up for that the service of the existing macro-cell becomes deteriorated in a building, and provides the service intended for the a specific user, thereby providing voice and data service of high quality. Further, the femto-cell can provide a new service not given in the macro-cell, and the spread of the femto-cell may cause fixed-mobile convergence (FMC) to be accelerated and an industrial-based cost to be reduced.
The femto BS may be installed by a home user or a small office home office (SOHO) user. The femto BS undergoes Plug & Play and is capable of providing service like that of the macro BS as well as an idle mode. Several tens to several hundreds of femto BSs may be installed within a cell zone of the macro BS, and the femto BS can improve service of a area without coverage instead of the existing relay and lighten load of the macro BS. However, a cell region of the femto BS may be overlapped with that of the macro BS or another femto BS, and thus interference may occur between the femto-cells or between the femto-cell and the macro-cell. In particular, if the femto BS and the macro BS use the same frequencies or neighbor frequencies, not only a user of the femto BS but also a user of the macro BS around the femto BS may be under serious interference. To reduce the interference of the femto BS, there has been proposed a fractional frequency reuse (FFR) method on a level of cells and transmission control of the femto BS, but there has not been proposed a method of reducing interference with respect to individual users of a cell boundary.
Accordingly, there is required a method of reducing interference with regard to individual users of the cell boundary in the femto BS.